[en] The in vivo behavior of (-)-[¹¹C]phenylephrine (PHEN) is compared with the structurally similar but monoamine oxidase (MAO)-resistant analog (-)-[¹¹C]-m-hydroxyephedrine (HED), which is an established heart neuronal marker. The chiral synthesis of PHEN has been achieved by direct methylation of (-)-m-octopamine with either ¹¹CH₃I or CF₃SO¹¹₃CH₃. These synthetic methods produced PHEN with a specific activity ranging from 500-1000 Ci/mmol, in a radiochemical yield of >50% (EOS) and with an enantiomeric purity of 94-96%. Biodistribution studies indicate the initial uptake of PHEN in rat heart is approximately half that of HED. Following PHEN injection, radioactivity egresses from the rat heart rapidly, with 50% washout occurring from 5 to 60 min. HED washout over this interval was less than 20%. The heart neuronal selectivity determined by desipramine blockade of the amine neuronal transporter was 75-77% compared to 92-95% for HED. Ring-labeled (-)-[³H]phenylephrine gave tissue-to-blood concentration ratios and heart clearance times very similar to PHEN. Rats pretreated with the MAO A inhibitor clorgyline showed higher levels of activity in the heart at 15 and 60 min. Tandem PET studies with PHEN and HED in the closed-chest dog provided excellent heart images with both tracers